New particulate relaxation agents responsive to electric fieldshave been prepared. To test them, cells have been built to allowthe optical observation of changes in particle properties whenelectric fields are applied and other cells have been constructed to permit NMR relaxation times and images to be measured undersimilar conditions. The mechanisms are still incompletelyunderstood. Preliminary studies have been carried out with specialprobes built for the SISCO 4.7T/33cm and the GE GN 300spectrometers. Optical microscopy shows that microelectrodestrigger complex shape and volume changes.New dendrimer-basedagents for blood pool contrast and targeting to cancer cells have been developed.